According to Fox News:
Trump has cited frustration over Rosenstein's handling of a FISA application involving former Trump campaign staffer Carter Page.
Deputy Attorney General Rod Rosenstein is expected to leave his role in the coming weeks, sources familiar with the matter told Fox News.
Rosenstein wants to ensure a smooth transition for incoming Attorney General nominee Bill Barr and plans to transition out of the job in the coming weeks, the sources said.
Rosenstein long thought of his role as a two-year position and the two-year mark is coming soon, officials close to the departing attorney general told Fox News.
Speculation of Rosenstein’s departure mounted after the firing of former Attorney General Jeff Sessions in November. He has, until recently, overseen Special Counsel Robert Mueller’s probe into whether Russia meddled in the 2016 presidential election and has been a target of Trump on Twitter.
Rosenstein is a Trump nominee to the Justice Department.